Substrate for mounting element and process for its production

ABSTRACT

To provide a substrate for mounting element having sulfurization resistance improved by increasing planarity of the surface of a thick conductor layer. 
     The substrate  10  for mounting element of the present invention has such a structure that on a surface of a LTCC substrate or ceramics substrate as an inorganic insulating substrate  1 , a thick conductor layer  2  is formed as an element connection terminal. The thick conductor layer  2  is made of a metal composed mainly of silver (Ag) or copper (Cu) and formed by printing and firing a metal paste. This thick conductor layer has its surface planarized by wet blast treatment to a surface roughness Ra of at most 0.02 μm. A Ni/Au-plated layer  3  is formed on the thick conductor layer  2 , so that the surface of the thick conductor layer  2  is completely covered without any space.

TECHNICAL FIELD

The present invention relates to a substrate for mounting element and aprocess for its production, particularly to a substrate for mountingelement excellent in sulfurization resistance, wherein planarity of asurface of a thick conductor layer formed on the substrate surface isgood, and a process for producing such a substrate.

BACKGROUND ART

In recent years, along with a tendency to a high brightness andwhitening of a light-emitting diode (hereinafter referred to as LED)element, a light-emitting device employing a LED element has been usedfor backlights of mobile phones or liquid crystal TVs or liquid crystaldisplays, generic illumination, etc. Accordingly, peripheral componentsof a LED element are also required to have higher performance. Forexample, as a substrate to mount a LED element, one made of a resinmaterial is used. However, such a resin substrate is likely to bedeteriorated by a heat or light accompanying higher brightness of a LEDelement. Accordingly, a study has been made to use a substrate made ofe.g. an inorganic insulating material.

Such an inorganic insulating material may, for example, be a ceramicssuch as alumina or aluminum nitride, or a low temperature co-firedceramics (LTCC) which is a composite of glass with a ceramics powdersuch as alumina. LTCC is one fired usually at a temperature of fromabout 800 to 1,000° C. which is lower than the firing temperature forusual ceramics and is prepared by laminating a prescribed number ofgreen sheets made of glass and a ceramics powder (such as an aluminapowder or a zirconia powder), integrating them by hot pressing, followedby firing. An inorganic insulating substrate made of such inorganicinsulating materials has a higher durability against a heat or light ascompared with a resin substrate and thus is prospective as a substratefor mounting a LED element.

On the surface of an inorganic insulating substrate, a thick conductorlayer is formed which is prepared by printing a paste composed mainly ofa conductor metal such as silver (Ag) or copper (Cu), followed byfiring. And, among such thick conductor layers, particularly terminalportions (electrodes) to be connected to the element are subjected tolamination plating (Ni/Au plating) of nickel (Ni) plating and gold (Au)plating to maintain the wire bonding property, the adhesion strength andthe weather resistance. By such Ni/Au plating, sulfurization resistanceis imparted to prevent a color change by a reaction of the thickconductor layer with a sulfur (S) content in the air, etc.

However, in recent years, a substrate to mount a LED element or thelike, is required to have sulfurization resistance, and withconventional plated thicknesses (Ni-plated thickness of from 3 to 5μm/Au-plated thickness of from 0.1 to 0.3 μm) required for the wiringbonding portions, there has been a problem that a color change to blackis observed at the Ni/Au-plated portion in a sulfurization test inaccordance with JIS-C-60068-2-43, thus failing to pass the sulfurizationtest.

By a research conducted by the present inventor, it has been found thatsuch a color change of the Ni/Au-plated portion is attributable toformation of nickel sulfide by a Ni-plated layer exposed to the surface.That is, a thick conductor of Ag or the like has grain boundary voidspaces or surface irregularities, and even if a Ni-plated layer isformed, irregularities will remain on its surface, and even ifAu-plating is applied as the uppermost layer, if it cannot completelycover the Ni-plated layer, the Ni-plated layer as an underlayer of theAu-plated layer will be exposed on the surface. And, such an exposedNi-plated layer and a sulfur (S) content will react to formblack-colored nickel sulfide.

Heretofore, as a technique to prevent such sulfurization (color change)of connection terminal portions, a method of applying a protective coatby e.g. a silicone resin on the Ni-plated layer, a method of forming athick Au layer by paste printing instead of plating, or a method ofincreasing the thickness of the Au-plated layer, has been known.Further, particularly in the case of a LTCC substrate, a method has beenknown wherein the particle size of the Ag powder to be used as aconductor to constitute the thick conductor layer is made small toimprove the sintering property thereby to reduce grain boundaries.

However, the method of forming a thick Au layer or increasing thethickness of the Au-plated layer has had a problem that the productioncost increases substantially. Further, in the case of the LTCCsubstrate, there has been a problem such that if the sintering propertyis improved by reducing the particle size of the Ag powder, timing inshrinkage by firing will not meet with the substrate whereby thesubstrate is likely to undergo warpage.

Further, as a technique to treat the surface of the conductor layerformed on a LTCC substrate to improve the plating property, a method hasbeen proposed wherein prior to a plating step, the surface of a LTCCsubstrate is subjected to wet blast treatment, and glass exposed on thesurface of the conductor layer is removed (e.g. Patent Document 1).

However, by this method, it was not possible to increase thesulfurization resistance by removing the grain boundary void spaces orsurface irregularities of the thick conductor (Ag) layer. That is,although Patent Document 1 does not disclose the conditions for the wetblast treatment in detail, the blast treatment to remove glass is one tobreak and remove glass as a hard substance in a short time by blasting.Under blast treatment conditions for such a purpose, it has beendifficult to fill spaces among the conductor (Ag) particles. And, it hasbeen difficult to remove the surface irregularities of the thickconductor (Ag) layer thereby to planarize (smooth) the layer surface tosuch an extent to make it possible to completely cover it with theAu-plated layer having a usual thickness.

PRIOR ART DOCUMENT Patent Document

Patent Document 1: Japanese Patent No. 4,089,902

DISCLOSURE OF THE INVENTION Object to be Accomplished by the Invention

The present invention has been made to solve the above-describedproblems, and it is an object of the present invention to provide asubstrate for mounting element having sulfurization resistance improvedby increasing the planarity of the surface of a thick conductor layerformed on an inorganic insulating substrate.

Means to Accomplish the Object

The substrate for mounting element of the present invention comprises aninorganic insulating substrate made of an inorganic insulating material,a thick conductor layer formed on the inorganic insulating substrate andmade of a metal composed mainly of silver (Ag) or copper (Cu), and anelectroconductive metal-plated layer formed on the thick conductorlayer, wherein the thick conductor layer has its surface planarized bywet blast treatment and has a surface roughness Ra of at most 0.02 μm.

In the substrate for mounting element of the present invention, theinorganic insulating substrate may be a low temperature co-firedceramics (LTCC) substrate. Otherwise, the inorganic insulating substratemay be a ceramics substrate. And, the ceramics substrate may containalumina or aluminum nitride as the main component. Further, theelectroconductive metal-plated layer is preferably a nickel (Ni)/gold(Au)-plated layer.

The process for producing a substrate for mounting element of thepresent invention comprises a step of forming a thick conductorlayer-provided substrate having a thick conductor layer made of a metalcomposed mainly of silver (Ag) or copper (Cu), on a surface of aninorganic insulating substrate made of an inorganic insulating material,a step of applying wet blast treatment to the thick conductor layer toplanarize the surface of the thick conductor layer to a surfaceroughness Ra of at most 0.02 μm, and a step of forming a nickel(Ni)/gold (Au)-plated layer on the thick conductor layer having thesurface planarized by the wet blast treatment.

In the process for producing a substrate for mounting element of thepresent invention, the above step of forming a thick conductorlayer-provided substrate may comprise a step of printing a metal pastecomposed mainly of silver (Ag) or copper (Cu) on a surface of asubstrate made of a glass ceramics composition comprising a glass powderand a ceramics powder, to form a conductor pattern, and a step of firingthe substrate having the conductor pattern formed to sinter the glassceramics composition and fire the metal paste thereby to form the thickconductor layer made of a metal composed mainly of silver (Ag) or copper(Cu). Further, the above step of forming a thick conductorlayer-provided substrate may comprise a step of firing a ceramicscomposition comprising a ceramics powder and a sintering aid to obtain aceramics substrate, a step of printing a metal paste composed mainly ofsilver (Ag) or copper (Cu) on a surface of the ceramics substrate toform a conductor pattern, and a step of re-firing the ceramics substratehaving the conductor pattern formed to form, from the metal paste, thethick conductor layer made of a metal composed mainly of silver (Ag) orcopper (Cu).

And, it is preferred that an abrasive to be used for the wet blasttreatment is a ceramics powder having a particle size of from 25 to 150μm, and a medium is water. Further, the mixing ratio of the abrasive ispreferably from 20 to 60 vol % based on the total amount of the abrasiveand the water. Further, in the above wet blast treatment, it ispreferred that a blast liquid comprising the abrasive and the water isjetted from a jet orifice of 8 mm in diameter, of a nozzle made of boroncarbide, and the pressure is from 1.2 to 1.8 kg/cm².

Further, in the present invention, the surface roughness Ra is onerepresented by JIS B0601 (1994), 3 “Definition and Representation ofDefined Arithmetic Mean Roughness”. In the present invention, thesurface roughness Ra was measured by SURFCOM 1400D (name of machine,manufactured by Tokyo Seimitsu Co., Ltd.).

Advantageous Effects of the Invention

According to the present invention, the thick conductor layer made of ametal such as silver (Ag) or copper (Cu) formed on the surface of theinorganic insulating substrate, is planarized (smoothed) by the wetblast treatment, and spaces among the thick conductor (Ag) particles arefilled and the surface roughness Ra is adjusted to be at most 0.02 μm,whereby the plating property is good, and the thick conductor surfacecan be completely covered even by a Au-plated layer having a usualthickness. Thus, it is possible to obtain a substrate for mountingelement, which is excellent in sulfurization resistance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating an example of thesubstrate for mounting element of the present invention.

FIG. 2 is a cross-sectional view illustrating another example of thesubstrate for mounting element of the present invention.

FIG. 3 is a cross-sectional view illustrating a third example of thesubstrate for mounting element of the present invention.

FIG. 4 is a cross-sectional view illustrating a fourth example of thesubstrate for mounting element of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Now, the embodiments of the present invention will be described indetail. FIGS. 1 and 2 are, respectively, cross-sectional viewsillustrating the substrate 10 for mounting element of the presentinvention. The substrate 10 for mounting element has an inorganicinsulating substrate 1 made of an inorganic insulating material, and onemain surface (the upper surface in Figs.) is a mounting surface 1 a onwhich an element (a semiconductor element) such as a LED element is tobe mounted. The inorganic insulating substrate 1 may be a lowtemperature co-fired ceramics substrate (LTCC substrate) made of asintered product of a glass ceramics composition comprising a glasspowder and a ceramics powder, or a ceramics substrate made of a sinteredproduct composed mainly of alumina or aluminum nitride. Further, theshape, thickness, size, etc. of the inorganic insulating substrate 1 arenot particularly limited. Further, the inorganic insulating substrate 1may be one having a flat-form as shown in FIG. 1, or one having such ashape that a side wall 1 b is provided along the periphery of thesubstrate to form a mounting surface 1 a in a cavity, as shown in FIG.2.

The raw material composition of the inorganic insulating material toconstitute the inorganic insulating substrate 1, the firing conditions,etc. will be described in the production process given hereinafter.

On the mounting surface 1 a of the inorganic insulating substrate 1, athick conductor layer 2 being a connection terminal (electrode) to beelectrically connected to an element such as a LED element, is formed.The thick conductor layer 2 is made of a conductor metal composed mainlyof silver (Ag) or copper (Cu) and is formed by printing a conductormetal paste by e.g. screen printing, followed by firing, as describedhereinafter. The thick conductor layer 2 is formed on the same plane asthe mounting surface 1 a. Here, in a case where the inorganic insulatingsubstrate 1 is a LTCC substrate, as shown in FIGS. 3 and 4, a concave 1c may be formed on the mounting surface 1 a, and the thick conductorlayer 2 may be formed on the bottom surface in the concave 1 c.

Such a thick conductor 2 has its surface planarized or smoothed(hereinafter referred to as planarization) by wet blast treatment andhas a surface roughness Ra of at most 0.02 μm. Further, on such a thickconductor layer 2 having the surface planarized, a Ni/Au-plated layer 3is formed to have a laminated structure comprising a nickel (Ni)-platedlayer and a gold (Au)-plated layer formed thereon, and the surface ofthe thick conductor layer 2 is thereby completely covered without aspace. If the surface roughness Ra of the thick conductor layer 2exceeds 0.02 μm, it tends to be difficult to completely cover thesurface of the thick conductor layer 2 by the Ni/Au-plated layer 3, andthe sulfurization resistance tends to be inadequate. The surfaceroughness Ra of the thick conductor layer 3 is more preferably at most0.01 μm.

Also on a non-mounting surface 1 d being a main surface on the sideopposite to the mounting surface 1 a of the inorganic insulatingsubstrate 1, a thick conductor layer 2 may be formed as an externalconnection terminal (electrode). In such a structure, also the surfaceof the thick conductor layer 2 formed on the non-mounting surface 1 d ispreferably planarized by wet blast treatment in the same manner as thethick conductor layer 2 formed on the mounting surface 1 a. That is,also the thick conductor layer 2 formed on the non-mounting surface 1 dis preferably planarized by wet blast treatment to have a surfaceroughness Ra of at most 0.02 μm, and has such a structure that aNi/Au-plated layer 3 is formed thereon to completely cover the surfaceof the thick conductor layer 2 without a space. Here, in FIGS. 1 to 4,symbol 4 represents a via conductor which electrically connects anelement connection terminal on the mounting surface 1 a and an externalconnection terminal on the non-mounting surface 1 d.

In the substrate 10 for mounting element of the present invention, thethick conductor layer 2 made of a metal such as silver (Ag) or copper(Cu) formed on the surface of the inorganic insulating substrate 1, isplanarized by wet blast treatment to a surface roughness Ra of at most0.02 μm, and the Ni/Au-plated layer 3 is formed thereon to completelycover the surface of the thick conductor layer 2 without a space,whereby no color change will take place in the sulfurization test, andthe sulfurization resistance is excellent.

Among the substrates 1 for mounting element of the present invention,the substrate 1 for mounting element having a LTCC substrate can beproduced as follows.

<Production of Substrate for Mounting Element having LTCC Substrate>

[Formation of Glass Ceramics Green Sheet]

Firstly, a glass ceramics green sheet is formed. This green sheet isformed by adding a binder and, as the case requires, a plasticizer, asolvent, etc. to a glass ceramics composition comprising a glass powderand a ceramics powder (hereinafter referred to as a ceramics powder forLTCC) to prepare a slurry, and forming the slurry into a sheet by e.g. adoctor blade method, followed by drying.

The glass powder is not necessarily limited, but one having a glasstransition point (Tg) of at least 550° C. and at most 700° C., ispreferred. If the glass transition point (Tg) is lower than 550° C., theafter-mentioned binder burn out is likely to be difficult, and if itexceeds 700° C., the shrinkage-initiating temperature tends to be high,and the dimensional precision is likely to deteriorate.

As the glass powder, for example, a glass powder comprising from 57 to65 mol % of SiO₂, from 13 to 18 mol % of B₂O₃, from 9 to 23 mol % ofCaO, from 3 to 8 mol % of Al₂O₃, and from 0.5 to 6 mol % in total of atleast one selected from K₂O and Na₂O, is used. The 50% particle size(D₅₀) of the glass powder is preferably from 0.5 μm to 2 μm. If D₅₀ ofthe glass powder is less than 0.5 μm, the glass powder is likely tocohere, and not only the handling tends to be difficult, but also ittends to be difficult to uniformly disperse it. On the other hand, ifD₅₀ exceeds 2 μm, an increase of the glass softening temperature or thesintering failure is likely to occur. Here, in this specification, theparticle size is a value obtained by a particle size analyzer of a laserdiffraction scattering method. As the particle size analyzer of a laserdiffraction scattering method, a laser diffraction particle sizeanalyzer (tradename: SALD2100 manufactured by Shimadzu Corporation) wasused.

As the ceramics powder for LTCC, one which has been commonly used forthe production of a LTCC substrate, can be used. For example, an aluminapowder, a zirconia powder, or a mixture of an alumina powder and azirconia powder may, for example, be suitably used. D₅₀ of the ceramicspowder is preferably at least 0.5 μm and at most 4 μm.

Such a glass powder and a ceramics powder are blended and mixed, forexample, so that the glass powder would be from 30 mass % to 50 mass %,and the ceramics powder would be from 50 mass % to 70 mass % to obtain aglass ceramics composition. To such a glass ceramics composition, abinder and, if required, a plasticizer, a solvent, etc. are added toobtain a slurry.

As the binder, it is possible to suitably use, for example, a polyvinylbutyral or an acrylic resin. As the plasticizer, it is possible to use,for example, dibutyl phthalate, dioctyl phthalate or butylbenzylphthalate. Further, as the solvent, it is possible to employ an aromaticor alcohol-type organic solvent such as toluene, xylene or butanol.Further, a dispersing agent or a leveling agent may also be used.

The glass ceramics green sheet thus formed is cut into a prescribed sizeby using a punch cutting die or a punching machine, and at the sametime, via holes for interlayer connection are formed by punching atprescribed positions.

[Printing of Metal Paste]

On the surface of the glass ceramics green sheet, a conductor metalpaste is printed by a method such as screen printing to form a non-firedconductor pattern. Further, a conductor metal paste is filled in theabove-mentioned via holes for interlayer connection, to form a non-firedinterlayer connection portion. The conductor metal paste may, forexample, be one prepared by adding a vehicle such as ethylcellulose and,as the case requires, a solvent, etc. to a metal powder composed mainlyof e.g. silver (Ag) or copper (Cu) to form a paste. As the metal powder,a silver (Ag) powder, a mixed powder of silver and palladium, or a mixedpowder of silver and platinum, is, for example, preferably used. Here,the adhesive strength between the conductor metal and the substrate canbe sufficiently secured by the glass component contained in the glassceramics green sheet, and it is preferred to use a metal paste having noglass frit incorporated, in order not to increase the electricalresistance (resistance value) of the conductor metal.

[Lamination and Filing of Glass Ceramics Green Sheet]

A plurality of green sheets having non-fired conductor patterns formedare overlaid one on another while adjusting their positions andintegrated by heating and pressing, followed by heating at a temperatureof from 500° C. to 600° C. to carry out binder burn out by decomposingand removing a binder such as a resin contained in the glass ceramicsgreen sheet. Thereafter, heating is carried out further at a temperatureof from about 800 to 1,000° C. to fire the glass ceramics compositionconstituting the glass ceramics green sheet. By this firing, the metalpaste formed inside and on the surfaces (the front and rear surfaces) ofthe glass ceramics substrate is simultaneously fired to form a thickconductor layer made of a metal composed mainly of silver (Au) or copper(Cu).

[Wet Blast Treatment]

The thick conductor layer formed on the surface of the LTCC substrate issubjected to wet blast treatment. That is, a blast liquid prepared bymixing an abrasive (blast material) with a liquid medium (such as water)is blasted (blown) to the thick conductor layer under high pressure. Bythis wet blast treatment, spaces among the conductor particles arefilled so that the surface of the thick conductor layer is planarized.By adjusting the particle size of the abrasive, the blast force(pressure) of the blast liquid, the treating time, etc., the surfaceroughness (Ra) of the thick conductor layer after the treatment can beadjusted to be at most 0.02 μm.

As the abrasive, a ceramic powder such as alumina or zirconia may, forexample, be used. In order to increase the blasting efficiency, it ispreferred to use a pulverized powder of an alumina powder. The particlesize of the abrasive is preferably within a range of from 25 to 150 μm.If the particle size of the abrasive is less than 25 μm, the abrasive islikely to enter into e.g. a groove for cutting of the LTCC substrate andconstitute a foreign matter which is likely to impair the mounting of anelement. On the other hand, if the particle size of the abrasive exceeds150 μm, it becomes difficult to efficiently blast the thick conductorlayer in the vicinity of the wall surface of a cavity being a mountingportion of an element. The 50% particle size (D₅₀) of the abrasive ispreferably within a range of from 80 to 100 μm. More preferred D₅₀ is 90μm.

The mixing ratio of the abrasive (blast material) and the liquid medium(such as water) is such that the abrasive will be from 20 to 60 vol %based on the entire amount of the blast liquid. If the mixing ratio ofthe abrasive is less than 20 vol %, the wet blasting efficiency islikely to be remarkably low and it becomes difficult to sufficientlyplanarize the surface of the thick conductor layer. On the other hand,if the ratio of the abrasive exceeds 60 vol %, the viscosity of theblast liquid tends to be too high, whereby the blasting efficiencyrather tends to deteriorate. The most preferred mixing ratio is suchthat the abrasive is 40 vol % and the water is 60 vol %.

Further, the flow rate (blast force) for jetting the blast liquid mixedin such a ratio, is preferably from 1.2 to 1.8 kg/cm². If the blastforce of the blast liquid is less than 1.2 kg/cm², the effect may beobserved for the removal of glass exposed on the surface of the thickconductor layer, but it becomes difficult to carry out sufficientplanarization so that the surface roughness Ra of the thick conductorlayer becomes at most 0.02 μm. Accordingly, it becomes difficult toimpart good sulfurization resistance. If the blast force of the blastliquid exceeds 1.8 kg/cm², an alumina powder being a blast material islikely to deposit on the surface of the thick conductor layer, wherebythe effect for planarizing the surface tends to be small.

In the wet blasting step, it is possible to adopt a method of jettingthe blast liquid from a jet orifice disposed at about 5 cm above thetransporting surface, towards the thick conductor layer of the LTCCsubstrate which is continuously transported by a belt conveyor. Thetransporting rate of the conveyor is preferably adjusted to be from 1 to1.5 m/min. If the transporting rate is less than 1 m/min, an aluminapowder being the blast material is likely to deposit on the thickconductor layer, whereby the effect to planarize the surface tends to besmall. If the transporting rate exceeds 1.5 m/min, the blasting effecttends to be small, and it becomes difficult to carry out sufficientplanarization for preventing sulfurization.

[Plating Step]

On the thick conductor layer planarized to a surface roughness Ra of atmost 0.02 μm by the wet blast treatment, Ni plating is carried out andthen Au plating is carried out to form a Ni/Au-plated layer. TheNi-plated layer is formed in a thickness of from 5 to 10 μm, forexample, by electrolytic plating using a nickel sulfamic acid bath. Thegold-plated layer can be formed in a thickness of from 0.2 to 0.5 μm,for example, by electrolytic plating using a gold potassium cyanidebath.

In the previous step, the thick conductor layer as the underlayer wassubjected to wet blast treatment to fill spaces among the conductor(e.g. Ag) particles thereby to smooth the irregularities and the surfaceis planarized to a surface roughness Ra of at most 0.02 μm, andaccordingly, the thick conductor layer can be completely covered by theNi/Au-plated layer having the above thickness. Thus, the Ni-plated layeris not exposed, and the sulfurization resistance is excellent, and inthe sulfurization test in accordance with JIS C-60068-2-43, it ispossible to obtain a Au-plated film free from a black-colored defect dueto precipitation of nickel sulfide on the surface of the Au-plated film.

Now, the process for producing the substrate 1 for mounting elementhaving a ceramics substrate, will be described.

<Production of Substrate for Mounting Element having Ceramics Substrate>

[Formation of Ceramics Green Sheet]

Firstly, a ceramics green sheet is formed. This green sheet can beformed by adding a binder and, as the case requires, a plasticizer, asolvent, etc. to a ceramics composition comprising a ceramic powder anda sintering aid to prepare a slurry, forming the slurry into a sheet bye.g. a doctor blade method, followed by drying.

As the ceramics powder, an alumina powder or an aluminum nitride powdermay be used. The 50% particle size (D₅₀) of the ceramics powder ispreferably from 0.5 μm to 2 μm. If D₅₀ of the ceramics powder is lessthan 0.5 μm, the ceramics powder is likely to cohere, and not only thehandling tends to be difficult, but also it becomes difficult touniformly disperse it. On the other hand, if D₅₀ exceeds 2 μm, sinteringdeficiency is likely to occur.

As the sintering aid, one which has been commonly used for theproduction of a ceramics substrate, may be used. For example, a mixtureof SiO₂ and an alkaline earth metal oxide, or a rare earth element oxide(particularly a Y₂O₃-type aid composed mainly of Y₂O₃) may be suitablyused. D₅₀ of the sintering aid is preferably from 0.5 μm to 4 μm.

Such a ceramics powder and a sintering aid are blended and mixed, forexample, so that the ceramics powder would be from 80 mass % to 99 mass%, and the sintering aid would be from 1 mass % to 20 mass %, to obtaina ceramics composition, and to such a ceramics composition, a binderand, as the case requires, a plasticizer, a solvent, etc. are added toobtain a slurry.

As the binder, it is possible to suitably use, for example, a polyvinylbutyral or an acrylic resin. As the plasticizer, it is possible to use,for example, dibutyl phthalate, dioctyl phthalate or butylbenzylphthalate. Further, as the solvent, it is possible to use an aromatic oralcohol-type organic solvent such as toluene, xylene or butanol.Further, a dispersing agent or a leveling agent may also be used.

The ceramics green sheet thus formed is cut into a prescribed size by apunch cutting die or a punching machine, and at the same time, via holesfor interlayer connection may be formed by punching at prescribedpositions.

[Firing of Ceramics Green Sheet]

The non-fired ceramics green sheet is heated at a temperature of from500° C. to 600° C., to carry out binder burn out by decomposing andremoving a binder such as a resin contained in the green sheet. In thecase of laminating non-fired ceramics green sheets, a plurality of themare overlaid one on another while adjusting their positions andintegrated by heating and pressing, and then, the above-mentioned binderburn out is carried out. Thereafter, heating is further carried out at atemperature of from 1,100 to 2,200° C. to fire the ceramics compositionconstituting the ceramics green sheet to obtain a ceramics substrate.

[Printing of Metal Paste]

On the surface of the ceramics substrate, a conductor metal paste isprinted by a method such as screen printing to form a non-firedconductor pattern. Further, a conductor metal paste is filled in theabove-mentioned via holes for interlayer connection to form non-firedinterlayer connection portions. As the conductor paste, one prepared byadding a vehicle such as ethylcellulose and, as the case requires, asolvent, etc. to a metal powder composed mainly of e.g. silver (Ag) orcopper (Cu) to form a paste, is used. As the metal powder, a silver (Ag)powder, a mixed powder of silver and palladium, or a mixed powder ofsilver and platinum, may, for example, be preferably used. Further, inorder to sufficiently secure the adhesive strength between the conductormetal and the ceramics substrate, a metal paste having a small amount ofglass flit incorporated, may be used.

[Firing (Re-Firing) of Metal Paste]

The ceramics substrate having the metal paste printed thereon is heatedat a temperature of from 500 to 1,000° C. to fire the metal paste formedinside (in via holes) and on the surfaces (the front and rear surfaces)of the ceramics substrate thereby to form a thick conductor layer madeof a metal composed mainly of silver (Ag) or copper (Cu).

[Wet Blast Treatment]

The thick conductor layer formed on the surface of the ceramicssubstrate is subjected to wet blast treatment. That is, a blast liquidprepared by mixing an abrasive (blast material) with a liquid medium(such as water) is blasted (blown) to the thick conductor layer underhigh pressure. By this wet blast treatment, spaces among the conductorparticles are filled to planarize (smooth) the surface of the thickconductor layer. By adjusting the particle size of the abrasive, theblast force (pressure) of the blast liquid, the treating time, etc., thesurface roughness Ra of the thick conductor layer after the treatmentcan be adjusted to be at most 0.02 μm.

Details of the type and particle size (D₅₀) of the abrasive in the wetblast treatment, the mixing ratio of the abrasive to the liquid medium(such as water), the flow rate (blast force) for jetting the blastliquid, the transporting rate of the substrate, etc. are the same asdescribed in the production of the substrate for mounting element havingthe LTCC substrate, and their description is omitted here.

[Plating Step]

On the thick conductor layer planarized to a surface roughness Ra of atmost 0.02 μm by the wet blast treatment, Ni plating is carried out andthen Au plating is carried out to form a Ni/Au-plated layer. TheNi-plated layer is formed in a thickness of from 5 to 10 μm, forexample, by electrolytic plating using a nickel sulfamic acid bath. Thegold-plated layer can be formed in a thickness of from 0.2 to 0.5 μm,for example, by electrolytic plating using a gold potassium cyanidebath.

In the previous step, the thick conductor layer as the underlayer wassubjected to wet blast treatment to fill spaces among the conductor(e.g. Ag) particles thereby to smooth the irregularities and the surfaceis planarized to a surface roughness Ra of at most 0.02 μm, andaccordingly, the thick conductor layer can be completely covered by theNi/Au-plated layer having the above thickness. Thus, the Ni-plated layeris not exposed, and the sulfurization resistance is excellent, and inthe sulfurization test in accordance with JIS C-60068-2-43, it ispossible to obtain a Au-plated film free from a black-colored defect dueto precipitation of nickel sulfide on the surface of the Au-plated film.

EXAMPLES

Now, the present invention will be described in detail with reference toExamples. However, it should be understood that the present invention isby no means restricted to such Examples.

Example 1

A substrate 10 for mounting element having a construction as shown inFIG. 1, provided with a LTCC substrate, was prepared.

Firstly, a glass ceramics green sheet for main body to prepare thesubstrate 10 for mounting element, was prepared. In the preparation, rawmaterials were blended and mixed so that SiO₂ became 60.4 mol %, B₂O₃15.6 mol %, Al₂O₃ 6 mol %, CaO 15 mol %, K₂O 1 mol % and Na₂O 2 mol %,and this raw material mixture was put into a platinum crucible andmelted at 1,600° C. for 60 minutes. Then, this molten state glass wascast and cooled. This glass was ground by a ball mill made of aluminafor 40 hours to obtain a glass powder for substrate main body. Here,ethyl alcohol was used as the solvent at the time of grinding.

40 mass % of this glass powder for main body and 60 mass % of an aluminapowder (tradename: AL-45H manufactured by Showa Denko K. K.) wereblended and mixed to prepare a glass ceramics composition. To 50 g ofthis glass ceramics composition, 15 g of an organic solvent (a mixtureof toluene, xylene, 2-propanol and 2-butanol in a mass ratio 4:2:2:1),2.5 g of a plasticizer (di-2-ethylhexyl phthalate), 5 g of polyvinylbutyral (tradename: PVK#3000K manufactured by DENKI KAGAKU KOGYOKABUSHIKI KAISHA) as a binder and a dispersing agent (tradename: BYK180manufactured by BYK Japan KK) were blended and mixed to prepare aslurry.

This slurry was applied on a PET film by a doctor blade method and driedto prepare a green ceramics sheet for main body which would have athickness of 0.15 mm after firing.

On the other hand, a conductive powder (tradename: S400-2 manufacturedby Daiken Chemical Co., Ltd.) and ethylcellulose as a vehicle wereblended in a mass ratio of 90:10 and dispersed in α-terpineol as asolvent so that the solid content would be 87 mass %. Then, kneading wascarried out in a porcelain mortar for 1 hour, and further, dispersionwas carried out three times by a three roll mill to prepare a metalpaste.

Through-holes having a diameter of 0.3 mm were formed in the greenceramics green sheet for main body at portions corresponding to viaconductors by means of a punching machine and filled with the abovemetal paste by a screen printing method to form non-fired via conductorpaste layers, and at the same time, a non-fired thick conductor layerwas formed, to obtain a glass ceramics green sheet for main bodyprovided with a thick conductor layer.

The glass ceramics green sheet for main body provided with a thickconductor layer, obtained as described above, was held at 550° C. for 5hours to carry out binder burn out and further held at 870° C. for 30minutes to carry out firing to prepare a substrate 10 for mountingelement for test.

In the obtained substrate 10 for mounting element, the thick conductorlayer corresponding to symbol 2 was subjected to wet blasting under thefollowing conditions.

The mixing ratio of the abrasive (blast material) and the liquid medium(water) was such that the abrasive was 40 vol % based on the entireblast liquid. Further, by adjusting the flow rate (blast force) forjetting the blast liquid mixed in such a ratio to be 1.5 kg/cm², theblast liquid was jetted from a jet orifice disposed at 5 cm above thetransporting surface, towards the thick conductor layer which wastransported continuously by a belt conveyor. The surface roughness Ra ofthe thick conductor layer was thereby made to be 0.01 μm. Here, thetransporting rate of the belt conveyor was adjusted to 1.2 m/min.

On the surface of the obtained thick conductor layer, a Ni-plated filmhaving a thickness of 7 μm was formed by electrolytic plating using anickel sulfate bath, and on its surface, a Au-plated film having athickness of 0.3 μm was formed by electrolytic plating using a goldpotassium cyanide bath. The substrate 10 for mounting element thusobtained was subjected to exposure for 100 hours by a sulfurization testin accordance with JIS C-60068-2-43, whereby a black-colored defect dueto precipitation of nickel-sulfide did not form on the surface of theAu-plated film.

EXAMPLE 2

A substrate 10 for mounting element having a construction as shown inFIG. 1, provided with a ceramics substrate, is prepared.

Firstly, a ceramics green sheet for main body to prepare the substrate10 for mounting element, is prepared. In the preparation, firstly, 96mass % of an alumina powder (tradename: AL-45H manufactured by ShowaDenko K. K.) and 4 mass % of a sintering aid (a talc powder containing65.8 mass % of SiO₂ and 34.2 mass % of MgO) were blended and mixed toprepare a ceramics composition. To 50 g of this ceramics composition, 15g of an organic solvent (a mixture of toluene, xylene, 2-propanol and2-butanol in a mass ratio of 4:2:2:1), 2.5 g of a plasticizer(di-2-ethylhexyl phthalate), 5 g of polyvinyl butyral (tradename:PVK#3000K manufactured by DENKI KAGAKU KOGYO KABUSHIKI KAISHA) as abinder and a dispersing agent (tradename: BYK180 manufactured by BYKJapan KK)) are blended and mixed to prepare a slurry.

This slurry is applied on a PET film by a doctor blade method, followedby drying to prepare a ceramics green sheet for main body which willhave a thickness of 1 mm after firing. Then, through-holes having adiameter of 0.3 mm are formed at portions corresponding to viaconductors by means of a punching machine.

Then, this ceramics green sheet for main body is held at 550° C. for 5hours to carry out binder burn out and further held at 1,500° C. for 60minutes to carry out firing to prepare a substrate made of alumina(alumina substrate).

On the other hand, an electroconductive powder (tradename: S400-2manufactured by Daiken Chemical Co., Ltd.) and ethylcellulose as avehicle are blended in a mass ratio of 90:10, and dispersed inα-terpineol as a solvent so that the solid content will be 87 mass %.Then, kneading is carried out in a porcelain mortar for 1 hour, andfurther, dispersion is carried out three times by a three roll mill toprepare a metal paste.

On the surface and in the through holes of the alumina substrate, themetal paste is filled by a screen printing method to form non-fired viaconductor paste layers, and at the same time, a non-fired thickconductor layer is formed to obtain a ceramics substrate provided with athick conductor layer.

Then, this ceramics substrate provided with a thick conductor layer isheld at 870° C. for 30 minutes to fire the metal paste thereby toprepare a substrate 10 for mounting element for test.

In this substrate 10 for mounting element, the thick conductor layercorresponding to symbol 2 is subjected to wet blast treatment under thefollowing conditions.

The mixing ratio of the abrasive (blast material) and the liquid medium(water) is adjusted so that the abrasive is 40 vol % based on the entireblast liquid. Further, by applying a pressure of 1.5 kg/cm2 to the blastliquid mixed in such a ratio by means of a nozzle having an orificediameter of 8 mm and made of boron carbide, the blast liquid is jettedfrom a jet orifice disposed at 5 cm above the transporting surface,towards the thick conductor layer which is continuously transported by abelt conveyor. The surface roughness Ra of the thick conductor layer isthereby made to be 0.01 μm. Further, the transporting rate of the beltconveyor is adjusted to 1.2 m/min.

On the surface of this thick conductor layer, a Ni-plated film of 7 μmis formed by electrolytic plating in a nickel sulfamic acid bath, and onits surface, a Au-plated film having a thickness of 0.3 μm is formed byelectrolytic plating in a gold potassium cyanide bath. The substrate 10for mounting element thus obtained is subjected to exposure for 100hours in a sulfurization test in accordance with JIS C-60068-2-43,whereby a black-colored defect due to precipitation of nickel sulfidewill not form on the surface of the Au-plated film.

In the foregoing, the embodiments of the present invention have beendescribed, but it is apparent that the present invention is by no meanslimited thereto, and various changes may be carried out within a rangedisclosed in Claims.

MEANING OF SYMBOLS

1: inorganic insulating substrate, 2: thick conductor layer, 3:Ni/Au-plated layer, 4: via conductor, 10: substrate for mounting element

The entire disclosures of Japanese Patent Application No. 2010-018925filed on Jan. 29, 2010 and Japanese Patent Application No. 2010-203104filed on Sep. 10, 2010 including specifications, claims, drawings andsummaries are incorporated herein by reference in their entireties.

1. A substrate for mounting element, which comprises: an inorganicinsulating substrate made of an inorganic insulating material, a thickconductor layer formed on the inorganic insulating substrate and made ofa metal composed mainly of silver (Ag) or copper (Cu), and anelectroconductive metal-plated layer formed on the thick conductorlayer, wherein the thick conductor layer has its surface planarized bywet blast treatment and has a surface roughness Ra of at most 0.02 μm.2. The substrate for mounting element according to claim 1, wherein theinorganic insulting substrate is a low temperature co-fired ceramicssubstrate.
 3. The substrate for mounting element according to claim 1,wherein the inorganic insulting substrate is a ceramics substrate. 4.The substrate for mounting element according to claim 3, wherein theceramics substrate contains alumina or aluminum nitride as the maincomponent.
 5. The substrate for mounting element according to claim 1,wherein the electroconductive metal-plated layer is a nickel (Ni)/gold(Au)-plated layer.
 6. A process for producing a substrate for mountingelement, which comprises: a step of forming a thick conductorlayer-provided substrate having a thick conductor layer made of a metalcomposed mainly of silver (Ag) or copper (Cu), on a surface of aninorganic insulating substrate made of an inorganic insulating material,a step of applying wet blast treatment to the thick conductor layer toplanarize the surface of the thick conductor layer to a surfaceroughness Ra of at most 0.02 μm, and a step of forming a nickel(Ni)/gold (Au)-plated layer on the thick conductor layer having thesurface planarized by the wet blast treatment.
 7. The process forproducing a substrate for mounting element according to claim 6, whereinthe step of forming a thick conductor layer-provided substratecomprises: a step of printing a metal paste composed mainly of silver(Ag) or copper (Cu) on a surface of a substrate made of a glass ceramicscomposition comprising a glass powder and a ceramics powder, to form aconductor pattern, and a step of firing the substrate having theconductor pattern formed to sinter the glass ceramics composition andfire the metal paste thereby to form the thick conductor layer made of ametal composed mainly of silver (Ag) or copper (Cu).
 8. The process forproducing a substrate for mounting element according to claim 6, whereinthe step of forming a thick conductor layer-provided substratecomprises: a step of firing a ceramics composition comprising a ceramicspowder and a sintering aid to obtain a ceramics substrate, a step ofprinting a metal paste composed mainly of silver (Ag) or copper (Cu) ona surface of the ceramics substrate to form a conductor pattern, and astep of re-firing the ceramics substrate having the conductor patternformed to form, from the metal paste, the thick conductor layer made ofa metal composed mainly of silver (Ag) or copper (Cu).
 9. The processfor producing a substrate for mounting element according to claim 6,wherein an abrasive to be used for the wet blast treatment is a ceramicspowder having a particle size of from 25 to 150 μm, and a medium iswater.
 10. The process for producing a substrate for mounting elementaccording to claim 9, wherein the mixing ratio of the abrasive is from20 to 60 vol % based on the total amount of the abrasive and the water.11. The process for producing a substrate for mounting element accordingto claim 9, wherein in the wet blast treatment, the blast force of ablast liquid comprising the abrasive and the water is from 1.2 to 1.8kg/cm².